1. Field of the Invention
The present invention generally relates to knee implant where the femoral component is derived directly from anatomic shapes, including the femoral trochlea, the lateral bearing surface, the medial bearing surface, and the intercondylar area between them.
2. Description of the Related Art
The first condylar replacement type of total knee replacement intended for cruciate resection was the Freeman-Swanson (Freeman, Swanson, Todd 1977), designed in the late 1960's. This used a roller-in-trough geometry to provide stability and a large area of contact to minimize the wear. Also in the late 1960's, Gunston (1971) designed a conservative total knee consisting of independent runners embedded in the femoral and tibial condyles. In the early 1970's, Seedhom (1974) designed a total knee based on anatomical knee specimens, where he directly replicated the femoral surfaces and the tibial surfaces with the menisci present. A similar concept was described by Ewald in a patent. The Total Condylar knee was designed in the early 1970's with partially conforming bearing surfaces in the frontal and sagittal planes In order to provide an appropriate combination of laxity and stability (Walker, Wang, Masse 1974; Insall, Ranawat, Scott, Walker 1976). The relative radii were calculated to provide similar mechanical characteristics to that of the anatomic knee. The Kinematic Stabilizer and Insall-Burstein designs added an intercondylar cam-post mechanism to prevent anterior femoral subluxation and posterior femoral displacement in high flexion (Walker & Sathasivam, 2000; Robinson R D 2005). Since then, these ‘posterior stabilized’ (PS) designs have been modified and refined, and are now in widespread use. Typically the sagittal and frontal geometries are defined by connecting radii generally resembling the Total Condylar, while the intercondylar cam-post is designed separately, usually contacting from mid-flexion to maximum flexion. In such designs, the lateral and medial condyles are often symmetric, providing no lateral or medial bias to the motion. While symmetric designs can use simply derived geometry for both the femoral and tibial surfaces, as well as for cams and posts, such an approach is more difficult if anatomic motion patterns are required. For any type of design where both of the cruciates are resected, the problem of replicating the constraints provided by these ligaments remains a challenge to this day. In particular, providing AP stability and rotational laxity throughout flexion, while inducing femoral rollback to achieve high flexion angles without posterior impingement, seems difficult to achieve with the bearing surfaces alone, even with a cam-post mechanism.
A further drawback of standard designs is that they only approximate the anatomical geometry, such that the metallic and plastic components do not closely replicate the hone and cartilage shapes which are removed at surgery. This results in altered ligament length patterns and muscle lever arms, which can cause instability, tightness in certain parts of the flexion arc, or inefficient use of muscles, or even pain such as in the patello-femoral joint.
More recently, designs have been produced which provide greater medial than lateral constraint. One design concept, the medial pivot, uses a ball-in-a-socket for the medial compartment, and surfaces of low constraint on the lateral side (Blaha 2004; Moonot, 2009). Another design, the Journey Knee (Reis, Victor, Bellemans 2006, Victor Bellemans 2006), has a more constrained medial side and a cam-pivot which results in more posterior femoral displacement laterally. These designs are intended to achieve more normal kinematics, a goal that is receiving more attention today in an effort to improve function, especially in more active patients. So far however, these designs do not replicate anatomic motion and laxity-stability characteristics, or have certain motion abnormalities in some patients. Hence there is still a need for a design that will allow close restoration of normal kinematics, and provides reliability and reproducibility.